The manufacture of metal components using a metal powder as the raw material, i.e., powder metallurgy, has been used for decades. Powder metallurgy is an excellent method of shaping metals into a predetermined design because of an efficient use of energy and materials. Powder metallurgy provides metal components of near net shape, and therefore is a common method of manufacturing large volumes of close tolerance metal components.
The manufacture of a metal component by powder metallurgy includes four basic steps to convert a metal powder into a metal component. Each step is controlled such that the finished metal component conforms to design specifications both within a single production batch and also between production batches.
The first step is preparation of a metal powder mixture. The metal powder mixture typically includes: (1) the metal powder being used as the material of construction, and (2) a lubricant. The metal powder can be a single metal species or can be a combination of different types of metal species. The metal powder particles typically are spherical in shape. The lubricant is added to minimize friction between the metal power and the tooling during a compaction, or pressing, step. The lubricant is present in an amount of up to about 2% by weight of the metal powder mixture.
After forming the metal powder mixture, the mixture is pressed in a die of predetermined shape. During the pressing operation, the spherical metal powder particles deform and interlock to form a compressed article, termed a "green compact," having a die fill ratio of about 2.3 to 1, or about 40% the original height of the metal powder height. As used here and hereafter, the term "die fill ratio" is defined as the ratio of the uncompressed metal powder height in the die to the height of the green compact. The shape of the green compact is determined by the geometry of the die. The green compact can be handled, but is fragile.
The density of the green compact (i.e., "green density") is determined primarily by the applied pressing load. The ability of the green compact to maintain its predetermined shape without cracking, fracturing, or crumbling during handling is referred to as the "green strength" of the compact. If green strength is too low, the green compact easily crumbles or cracks when removed from the die, which makes manufacture into a methyl component difficult to impossible.
After pressing, the green compact is subjected to an elevated temperature to form a metal component. The green compact is heated at a sufficiently high temperature and for a sufficient time to decompose, or pyrolyze, the lubricant, and to increase the density and strength of the metal component.
Conventionally, the green compact is heated in steps, initially to a first temperature to pyrolyze the lubricant, then to a second higher temperature to increase the density and strengthen the metal component, i.e., to sinter the metal component. A typical sintering furnace comprises a continuously running mesh belt which carries the green compacts through the furnace. Heating cycle times typically are about 1 to 3 hours, with 20 to 60 minutes at a sintering temperature in excess of 1000.degree. C. The sintered metal component, after cooling, then is subjected to optional secondary operations, such as deburring, to provide the final finished metal component.
The strength of a metal component is directly related to the density of the metal component, which in turn is directly related to the density of the green compact. Therefore, investigators have continually searched for ways to increase the density of both the green compact and the metal component to approach 100% theoretical density. As used here and hereafter, the term "100% theoretical density" is defined as the density of the metal, metals, and/or alloys forming the metal component. "Percent (%) theoretical density" is defined as the ratio of green compact density, or metal component density, to the density of the metal, metals, and/or alloys from which the green compact or metal component is manufactured.
Metal components manufactured by the above-described traditional powder metallurgy process, and using metal powder particles having spherical or near spherical geometry, have a theoretical density of about 88% to about 92%. Metal components having a density in this range often exhibit low strength, and are susceptible to corrosion due to the porosity of the metal component. Such metal components are unsuitable for many practical applications because they are subject to failure. In some instances, these relatively low density metal components are used in high load applications but are oversized to withstand use conditions.
One method investigators found to increase the density and improve the physical properties of a metal component manufactured by powder metallurgy was to press the metal component a second time, after sintering. The repressing step is termed "sizing" or "restriking." Then, if desired, the sized metal component can be resintered. Typically, repressing and resintering provides a metal component having a density up to about 95% of theoretical density. The extra processing steps of repressing and resintering are costly and time-consuming, and only minimally improve the density and physical properties of the metal component over traditional powder metallurgy processes.
Another powder metallurgy technique used to increase the density of the metal component is "warm" pressing the mixture of metal powder and lubricant at a temperature up to about 370.degree. C., and usually at about 150.degree. C. to about 260.degree. C. "Warm" pressing provides a green compact having a higher density than a green compact prepared by traditional powder metallurgy techniques which utilize "cold," or ambient temperature, pressing. Sintering a warm-pressed green compact provides a metal component having a density up to about 95% of theoretical density, and typically about 85% to about 94% of theoretical density. The method of pressing at an elevated temperature is disclosed in Rutz et al. U.S. Pat. No. 5,154,881, for example. A primary disadvantage of "warm" pressing is the increased cost of the metal component.
Other methods, such as hot isostatic pressing, also have been used to increase the density, and decrease the porosity, of metal components manufactured by powder metallurgy. An exemplary technique is disclosed in James et al. U.S. Pat. No. 5,080,712. Each of the above-described techniques is more costly than a traditional powder metallurgy process, but provided metal components having a density no greater than about 96% of theoretical density.
Workers in the art also investigated whether the shape of the metal powders affected the density of green compacts and metal components prepared by powder metallurgy. Conventionally, metal powder particles used in powder metallurgy are spherical or near-spherical in shape.
In powder metallurgy, micron-sized, spherical metal powder particles are blended with a die lubricant and compacted into a predetermined shape. The amount of lubricant used with spherical metal powder is about 0.25% to about 2%, and typically about 0.5% to about 1%, by weight of the metal powder mixture. After pressing, a green compact containing the spherical metal powder has a green density less than 90% of theoretical density. This relatively low green density is attributed to: (1) the resistance of spherical metal powder to efficiently compress to high densities in a die, (i.e., spheres inherently resist compaction and arrays of spheres have substantial void spaces between the spheres), and (2) the relatively higher volume occupied by the low density lubricant (which decreases the overall density of the green compact). During heating and sintering, the lubricant is pyrolyzed and the density of the resulting metal component is increased, but typically to less than 93% of the theoretical density of the metal or metal alloy. The low density of the metal component adversely affects performance.
Spherical metal powder particles require relatively large amounts of lubricant because each powder particle must be coated with a minimum amount of lubricant, and spherical powder particles have a large surface area to weight ratio. However, it is desirable to minimize the amount of lubricant in the metal powder mixture in order to minimize the die volume occupied by the lubricant, and thereby increase the density of the green compact. Investigators attempting to minimize the amount of lubricant present in the metal powder mixture have addressed the morphology, i.e., the size and shape, of the metal powder particles.
With respect to size of the metal powder particles, because each metal powder particle requires a minimum thickness of lubricant coating, the present investigators theorize that reducing the surface area-to-weight ratio of the metal powder particles prior to pressing would reduce the amount of lubricant needed to coat the metal powder. The reduced amount of lubricant would result in an increased green density, and, subsequently, an increased density of the finished metal component.
Increasing the size of spherical metal powder particles reduces the surface area-to-weight ratio of metal particles. For example, an idealized 20 micron spherical iron particle has a surface area-to-weight ratio of about 400 square centimeters per gram (cm.sup.2 /g). Increasing the diameter of the spherical powder particle by a factor of two, or to 40 microns, reduces the surface area-to-weight ratio to about 200 cm.sup.2 /g, or about one-half. The larger spherical powder size can be expected to reduce the amount of lubricant required to lubricate the die. However, increasing the size of the spherical powdered metal does not appreciably increase the density of the final metal component because spherical metal powders of any size do not readily compress to provide a high density green compact or metal component.
In addition, the green strength of the green compact is reduced when larger spherical metal powders are used. Thus, preferred metal powder particles have a shape that is more easily compressed than spherical metal powders, have a low surface area-to-weight ratio, and exhibit a sufficient green strength to facilitate handling of the green compact.
Various publications have addressed the effects of powder particle size, powder particle shape, lubricant content, sintering temperature and pressing techniques on the density of a metal part manufactured by powder metallurgy. These publications include:
S. Suh et al., "A Study of Compressibility, Green and Sintered Strength of Iron Powders," Conference Proceedings, Advances in Powder Metallurgy-1991, Vol. 5, P/M Materials, pages 151-160 (1991); PA1 H. H. Hausner, "Correlation Between Characteristics of Powders and Pores and Their Effect on the Properties of P/M Materials," Conference Proceedings, P/M-82 in Europe, International Powder Metallurgy Conference, pages 569-575 (1982); and PA1 H. I. Sanderow, "High Temperature Sintering of Ferrous P/M Components," New Perspectives in Powder Metallurgy, Vol. 9, High Temperature Sintering, pages 15-34 (1990). PA1 R. F. Krause, "AC Magnetic Characteristics of Cores Made from Pressed, Annealed, and Repressed Rectangular Steel Particles," J. Appl. Phys., 57(1), pages 4255-4257, Apr. 15, 1985; PA1 Pavlik et al. U.S. Pat. No. 3,948,561; PA1 Pavlik et al. U.S. Pat. No. 4,158,561; PA1 Reynolds et al. U.S. Pat. No. 4,158,580; PA1 Krause et al. U.S. Pat. No. 4,158,581; PA1 Krause et al. U.S. Pat. No. 4,158,582; and PA1 Krause et al. U.S. Pat. No. 4,265,681.
The following patents and publication disclose the pressing of elongated rectangular-shaped particles into magnet materials. The metal particles were thin, elongated parallelopipeds having a substantially rectangular cross section and a width to height ratio of about 1.5:1 to about 4:1.
The particle shape disclosed in the above-identified publication and patents provides green compacts having high green densities. However, these rectangular cross section particles have a poor flowability and a poor die fill ratio (i.e., about 3.5 to 1 or greater). As a result, elongated metal particles having a substantially rectangular cross section are difficult to compact during pressing.
In particular, metal particles having a poor die fill ratio of greater than 3 to 1 require deep pressing dies (i.e., have a substantial height), and poor flow characteristics. The poor flow characteristics of the particles having a substantially rectangular cross section results in long die fill times and high production costs. Also, high die abrasion (i.e., short die life) can occur when pressing metal powders having a poor die fill ratio.